Transformer bushing



Patented Jan. 7, ,1941,

UNITED STATES PATENT OFFICE TRANSFORMER BUSHING Pennsylvania Application October 18, 1938, Serial No. 235,611

Claims.

The present invention relatesto bushings for insulating the conducting leads of high-tension electrical apparatus, such as transformers and circuit breakers, and has particular reference to 5 an improved cap for closing the exposed end of such bushings.

Bushings of the type contemplated usually comprise an elongated cylindrical member of insulating material, such as porcelain, through which the conductor or lead from the apparatus extends to be connected at the upper end of the casing to a terminal which, in turn, is to be connected to an electrical circuit. The conducting lead in many cases is surrounded by a hollow metallic stud or the stud itself may be used as the conductor. In either event, by reason of the diii'erence in the coefficient of 'expansion between the metallic and porcelain parts, it is difiicult to maintain the bushing fluid tight and to prevent breathing upon change in temperature. The bushing is usually lled with a dielectric iluid, which may be oil or a gum, and upon breathing cycles moisture may be drawn into the bushing with resultant damage.

Further, it is usually undesirable to have communication between the hollow stud and the interior of the bushing, and this is particularly difcult because the thermal expansion and contraction of the hollow stud longitudinally thereof makes it necessary to terminate the stud within the bushing and allow space to permit such expansion and contraction.

It is an object of the present invention to provide a bushing terminal construction which is comparatively simple as compared with prior art structures and which may be assembled or disassembled with a minimum expenditure of time.

It is another object of the invention to provide.

a bushing terminal cap, in conjunction with a bushing having a hollow metallic stud, in which means is provided to prevent communication between the interior of the stud and the interior of the bushing in spite of contraction and elongation of the stud under the influence of temperature changes.

Other objects of the invention will be apparent from the following description inconjunction with the accompanying drawing, in which:

Figure 1 is a view in elevation with parts in section and parts removed, of a. bushing assembly of the typel to which the present invention is applicable, and

Fig. 2 is an enlarged view in vertical section of the cap oi' the bushing shown in Fig. 1, a part 55 of the bushing being broken away.

Referring to Fig. 1, the bushing may comprise a cylindrical casing 2 of dielectric material, such as porcelain, having a plurality of flanges 4 in the usual manner. The casing 2 rests upon a metallic supporting collar B, being spaced therefrom by a. gasket 8. The supporting collar 6 is provided with a flange suitably bolted to a casing I0, only a portion of which is indicated in section, which may be the metallic casing of an oil-immersed transformer, circuit breaker, or the like. The-upper end oi' the bushing is surmounted by a cap I2, shown in greater detail in Fig. 2, which rests upon-the end face of the porcelain casing 2, being spaced therefrom by gaskets of diilerent compressibility so that one acts as a' stop for the other. The top of the cap i2 is surmounted by a terminal I6 to which the external electrical circuit is to be connected.

By way of example the present invention is intended to be used with a bushing of the condenser vtype in which alternate layers of conducting and insulating material are wound upon a hollow metallic stud. In Fig. l, the condenser wrapping is indicated at I8 and the hollow stud at 2U. It is to be understood that the condenser wrapping is wound tightly on the stud 20 and is substantially integral therewith. The wound condenser is tightly tted within the metallic collar 6 in a suitable manner, such as shown in the patent to E. E. Arnold et al. No. 2,088,502, issued July 27, 1937. It is intended that the condenser wrapping I8 be of such tight fit within the collar 6 that an oil-tight joint therebetween is obtained to effectively close the bottom of the bushing structure. It is intended, however, that the invention is applicable to bushings oi' other than the condenser type, in which event the bottom of the bushing. will be closed in some other suitable and usual manner, as by a porcelain shell enclosing the lower end of the bushing and having a plate hermetically sealing its lower end maintained in position by engagement with the bushing stud.

Referring to Fig. 2, the cap i2 comprises a main body portion of substantially cup-shape having a base portion proportioned to rest upon the end face of the insulating shell or casing 2. The body portion is provided with a central aperture through which loosely extends the upper end of the metallic stud 20, the latter terminating within the confines of the cap. The cap is provloed with a cover 22 suitably bolted thereto to effect a fluid-tight Joint and has a central extension 2l surmounted by a terminal 26 also-*se" cured to the extension 2l in a fluid-tight marmer.

The conducting lead 2l from the electrical apparatus within the tank Il is drawn up through the stud and suitably secured to a block threaded to the interior of the cover extension 24, with a nut l2.

Upon variations in temperature, relative movement between the porcelain shell 2 and the stud 20 will occur. The relative movement is quite small, but unless provision is made for it, the destructive forces resulting will, upon contraction of the stud, tend to crush the porcelain or crack the metallic parts of the bushing, and upon elongation, the various gasket joints may open, destroying the fluid-tight character of the bushing. In order to take care of this situation, a fitting l( is threaded to the upper end of the stud 2l, and then preferably brazed to it, and is provided with a plurality of apertures through which bolts Il extend, terminating at their lower ends in sockets 3l cast into the base of the cap i2. It is intended that the fitting J4 is preferably in the form of a spider and that the bolts I! will extend loosely through their respective apertures. The number oi bolts and springs required must be determined by the size oi the bushing, four being indicated, by way oi' example, in Fig. 2.

Each bolt is provided with a square flat plate Il adjacent its lower extremity to constitute the seat for a helical spring lll which surrounds the boit. The upper end of the spring is in engagement with the-under face of the ntting 34. B y this means, when the springs l0 are under compression, they will transmit a force from the stud 20 through the fitting u and bolt 36 to the base of the cap i2, which will maintain the gaskets .i4 under compression, as well as the gasket 8 at the lower end of the porcelain shell. The desired pressure may be maintained by adjustment of the bolts I., the plates I8 being square will engage the side wall of the cap and 'not turn with the bolts. The degree of compression of the springs I0 should be so chosen that they will always be under compression in spite of elongation of the stud 20 throughout the normal temperature range encountered in service. and throughout such range the springs are positively retained in operative position by the bolts ll.

The application of this phase of the invention to bushings in which the stud may be solid should be obvious.

In order to prevent communication between the interior of the stud 20 and the cap I2, a tubular iiexible member u of a fluid-impervious material is secured to the upper end of the stud 2U in fluid-tight manner by means of a clamp 4I. and the upper end of the flexible member Il is clamped between a depending conical flange Il on the cover 22 and the lower beveled edge of clamping member 5U, which is actuated to clamping position, as indicated by screwing down block 30 to an extent necessary to obtain a hermetic seal.

The flexible member N may be of any desired material so long as it is fluid-impervious, retains its flexibility over a reasonable period, and will not deteriorate under the influence of the oil, gum or other compound with which the bushing is usually filled. There are on the market many types of synthetic rubber, and fabric impregnated therewith which are suitable for this purpose, and I have found that one of such materials which is satisfactory is sold under the trade name Duprene.

By means of the construction described, it

should be apparent that the invention contemplates an unusually simple construction but at the same time takes care of the many severe conditions encountered in the normal use of bushings oi this character.

Also, the construction results in a simple assembly operation involving no soldered joints and a minimum of parts. In such assembly operation, when the cap I2, with the cover 22 removed, is placed in position, the fitting 3| may readily be screwed into place and the bolts 3B inserted with the springs 4l and plates 38 before the springs are placed under compression, The bolts Il may then be screwed down to place the springs Il under the desired degree of compression. This may be accomplished by a simple wrench rather than having to resort to expensive jigs to place the springs under compression before they are placed in position in the cap. The lower end of the flexible member M may then be clamped to the top oi' the stud 2|! by means of the clamp 43, and it is of sufllcient length that if the cover 22 were in position, it would extend into the cavity defined by the depending flange ll on the under-side of the cover. The clamping member III is inserted into the end of the iiexible member but not sumciently to deform such member, and the cover is then placed in proper position and bolted. The flexible member now extends into the cover cavity and the clamping member extends into the cover extension 2|. 'Ihe block 30 is then screwed down to expand the upper end of the flexible member M into contact with the ange I6 to complete the clamping and sealing action. The conductor 2l is then ished through and secured to the position shown and the terminal member 2l may then be bolted in position to complete the assembly. The disassembly for inspection or repairis equally simple.

Quite obviously, 4the design described and shown may be varied to take care of various conditions encountered in service, and it is intended that the invention is not to be limited except as imposed by the appended claims.

I claim as my invention:

i. In a bushing for high tension electrical apparatus including a cylindrical casing of insulating material having a' stud extending axially therethrough and projecting beyond an end thereof, said stud having a portion thereof fixed with respect to a portion of said casing, and a cap enclosing the projecting portion of said stud and having a be proportioned to seat on the atijacent end face of said casing; means lor securing said cap in such position comprising a fitting secured to said stud, a member extending loosely 'through a portion of said fitting into engagement with the base of said cap, and adjustable means reacting between said fitting and member for forcing said member against said cap base.

2. In a bushing for high tension electrical apparatus including a cylindrical casing of insulating material having a stud extending axially therethrough and projecting beyond an end thereof, said stud having a portion thereof fixed with respect to a portion of said casing, and a cap enclosing the projecting portion of said stud and having a base proportioned to seat on the adjacent end lace of said casing; means for securing and cap in such position comprising a fitting secured to said stud, a member extending loosely through a portion of said tting into engagement with the base of said cap, and adjustable resilient means under compression reacting between said fitting and member for biasing said member into engagement with said cap base.

3. In a bushing for high tension electrical apparatus including a cylindrical casing of insulating material having a. stud extending axially .therethrough and projecting beyond an end thereof, said stud having a portion thereof fixed with respect w a portion of said casing, and a cap enclosing the projecting portion of said stud and having a .base proportioned to seat on the adjacent end face of said casing; means acting between said stud and cap base for maintaining said cap in such position comprising a helical spring having one end fixed with respect to said stud and movable therewith, and a seat for the other end of said spring including means engaging said cap base and adjustable axially of the spring to adjustably compress the spring.

4. In a bushing for high tension electrical apparatus including a cylindrical casing of insulating material having a stud extending axially therethrough and projecting beyond an end thereof, said stud having a portion thereof fixed with respect to a portion of said casing, and a 'cap enclosing the projecting portion of said stud and having'a base proportioned to seat on thev adjacent end face of said casing; means acting between said stud and cap base for maintaining said cap in such position comprising a helical spring having one end iixed with respect to said stud and movable therewith, means for cornpressing said spring comprising a seat for the other end thereof, and means for adjustably positioning said seat axially of the spring including means bearing upon said cap base to transmit the expansive force of the spring against Athe cap base.

5. In a bush-ing for high tension electrical apparatus including a cylindrical casing of insulating material having a stud extending axially therethrough and projecting beyond an end thereof, said stud having a portion thereof fixed with respect to a portion of said casing, and a cap enclosing the projecting portion of said stud and having a base proportioned to seat .on the adjacent end face of said casing; means for securing said cap in such position comprising a fitting secured to said stud, a bolt extending loosely 'through an aperture in said fitting parallel to said stud and into engagement with said cap base,

a helical spring surrounding said bolt and in engagement at one end with said fitting, and means for adjustably compressing said spring including a seat for the other end thereof threaded to said bolt.

ALEXANDER A. SKVORTZOFF. 

